The invention relates to a measuring instrument and a measuring arrangement having at least one measuring instrument.
In the applications which are common in measurement and control engineering, for example in the monitoring, control and/or automation of complex processes, it is usual for a number of measuring instruments, for example pressure, temperature, flow and/or level measuring instruments, to be in use at the same time.
A measuring instrument generally comprises a sensor, which registers a physical measured variable and converts it into an electrical variable, and electronics which convert the electrical variable into a measurement signal. The measuring instruments have to be connected individually, that is to say they have to be supplied with power and the measurement signal has to be fed to a higher-order unit. The core of the higher-order unit is usually a control and/or regulating unit, which registers the measurement signals, evaluates them and supplies display, control and/or regulating signals for the monitoring, control and/or automation of a process as a function of the instantaneous measured values. Examples of this are programmable logic controllers (PLC), distributed control systems (DCS) or personal computers (PC).
In order to keep the work which is entailed during the installation of the measuring instrument to a low level, in measurement and control engineering use is preferably made of measuring instruments having only one pair of lines, via which both the supply to the measuring instrument and its signal transmission take place. These instruments are often referred to as two-measuring instruments are fed with 12 V, and the measuring instrument controls a current flowing via the pair of lines as a function of an instantaneous measured value. The measurement signal is a signal current in the case of these measuring instruments. According to a standard which is common in measurement and control engineering, the signal current is set to values between a minimum signal current of 4 mA and a maximum signal current of 20 mA, depending on the instantaneous measured value.
Since both the supply and the signal transmission take place via the pair of lines, given a feed voltage of 12 V and a signal current of 4 mA, there is only a power of 48 mW available to the measuring instrument. This is completely adequate for a very large number of measuring instruments. In large plants, therefore, terminal blocks are usually provided which have a large number of identical pairs of terminals for these pairs of lines to be connected to the higher-order unit. As a result of this standardization of the method of connection, a large number measuring instruments can be connected up very simply and quickly and therefore cost-effectively. Since all the pairs of lines and all the pairs of terminals are identical, wiring errors are virtually ruled out.
However, there are also measuring instruments, such as highly accurate level measuring instruments operating with microwaves, level measuring instruments operating with ultrasound or flowmeters, for which this low power is not adequate. In order that these measuring instruments can nevertheless be used in conjunction with the previously described standard, these measuring instruments usually have two pairs of lines. The measuring instrument is supplied via one of the pairs of lines, and a signal current corresponding to the previously described standard flows via the other pair of lines. For the supply, it is usually necessary to connect a transformer and a rectifier to the normal power line, which carries 230 V alternating voltage, for example, in order for example to provide a supply voltage of usually 24 V DC for the measuring instrument. This is very complicated, and there is the risk that the two pairs of lines can be transposed during the connection of the instrument.
It is an object of the invention to specify a measuring instrument which can be electrically connected, very simply and without errors, to a higher-order unit.
To this end, the invention consists in a measuring instrument to be connected to a higher-order unit having at least a first and an identical second pair of terminals, which comprises:
a first first [sic] pair of lines, to be connected to the first pair of terminals,
via which a signal current flows during operation,
the signal current being a measure of an instantaneous measured value, and
a second second [sic] pair of lines, to be connected to the second pair of terminals,
via which a supply current flows during operation,
whose value is greater than or equal to a minimum signal current and less than or equal to a maximum signal current.
According to one embodiment of the invention, the supply current and at least a proportion of the signal current are available to supply the measuring instrument.
According to a further embodiment, the minimum signal current is 4 mA and the maximum signal current is 20 mA.
According to a further embodiment, a current/voltage limiter is connected to the input side of each pair of lines.
According to a further embodiment, the first pair of lines is connected to a first circuit and the second pair of lines is connected to a second circuit, and the first and the second circuits are galvanically isolated from each other.
In addition, the invention consists in a measuring arrangement having at least one measuring instrument according to the invention, in which the higher-order unit comprises a control and/or regulating unit, in particular a programmable logic controller (PLC), a distributed control system (DCS) or a personal computer (PC).
According to one embodiment of the measuring arrangement, the higher-order unit has one or more batteries of transmitter feed units, at least one battery having at least two transmitter feed units and each transmitter feed unit having a pair of terminals.
According to a further embodiment, each battery of transmitter feed units is connected to the control and/or regulating unit via a bus access circuit and a bus line in order to transmit the measured values from the measuring instruments connected thereto.